It was easier to make the sides solid, and not mess with fabricating tool boxes. Now, I wouldn't mind having them, if I knew that I could make them water tight. The toolboxes on my flatbed trailer let water in past the gaskets, and that's my only real experience with them.
If I can come across some 19.5 alcoas, or possibly the new 17" alcoas that are being sold with the new lonestar model Dodge trucks, I'd be inclined to install them.